1. Field of the Invention
This invention relates to radio frequency ranging apparatus. More specifically, the apparatus relates to measurement of the two way range between a ground station and a remote (airborne) station and the simultaneous measurement of offset of the station clocks.
2. Description of the Prior Art
Two way radio frequency ranging systems are not new and such systems have been described in Jet Propulsion Laboratory Technical Report 32-1475 published July, 1970.
This article explains that ranging systems are conveniently assigned into classifications depending on their method of coding. One such method involves utilization of a pseudonoise code of sufficient period to resolve range ambiguities and a bit length short enough to provide the required resolution. The most desirable selection of a pseudonoise code requires long acquisition times. Several methods are suggested which would reduce acquisition time even though the complexity of the hardware implementation would be increased. A multicomponent combinatorial unit has been suggested employing component sharing at the cost of power transmission loss. The loss of power can be overcome by employing sequentially transmitting components; however, sequential use of components increases hardware cost and increases the time required for transmission.
The use of pseudonoise spread spectrum codes in data links permit weak spectral signals to be transmitted and decoded accurately after being received at a remote station. The use of continuous bandwidth pseudonoise spread spectrum codes permits processing gain enhancement at the receiver. It is important that the data link code cannot be decoded or jammed and the longer the binary sequence the more difficult it is for others to intercept and decode the sequence.
As the binary sequence is increased the acquisition time also increases. If some outage or interruption occurs between data links reacquisition must be established, which under hostile or unfavorable conditions, could be more difficult that original acquisition under favorable non-hostile conditions.
Transmission of information employing pseudonoise spread spectrum codes, whether continuous or discontinuous is dependant on having a replica of the sequence being transmitted available at the receiving station to be employed to demodulate the incoming waveform. Acquisition or reacquisition of the transmitted waveform requires that a replica of the transmitted sequence be locked on or synchronized in phase with the incoming sequence waveform.
When the ground station is receiving information from the remote station, it too must be provided with means for producing a replica of the transmitted sequence to be employed to demodulate the incoming waveform being transmitted from the remote station.
Heretofore, two way ranging has been performed through coherent turn-around of a marker signal. In coherent turn-around the demodulating replica at the remote station must be synchronized and in phase with the received waveform. The received marker signal is processed (i.e., turn-around) and re-transmitted back to the ground station where it is demodulated employing a replica sequence of the code. This replica sequence must also be locked on the transmitted waveform from the remote station. When coherent turn-around is required for ranging measurements. The remote station must acquire and lock on the original signal. After the remote station has acquired the ground station signal it may then transmit the processed signal back to the ground station, which must also acquire the retransmitted signal. Coherent turn-around requires that both stations be locked on to the waveform being transmitted from the other station.
It is known that range measurement between stations can be calculated by measuring the time an electro-magnetic signal takes to travel from a ground station to a remote station and return to the ground station. A complete knowledge of the delays encountered by the transmitted signal must be either known or can be accurately estimated.
It would be extremely desirable to provide a ranging system which is secure and not easily jammed. Further, it would be desirable to provide a ranging system which could use the structural elements of the data link already present at the stations and would operate in a manner which reduces acquisition time.
It would be extremely desirable to be able to measure the clock offset between clocks in different stations.